Silicon wafers or sheets may be used in, for example, the integrated circuit or solar cell industry. Demand for solar cells continues to increase as the demand for renewable energy sources increases. The majority of solar cells are made from silicon wafers, such as single crystal silicon wafers. Currently, a major cost of a crystalline silicon solar cell is the wafer on which the solar cell is made. The efficiency of the solar cell, or the amount of power produced under standard illumination, is limited, in part, by the quality of this wafer. As the demand for solar cells increases, one goal of the solar cell industry is to lower the cost/power ratio. Any reduction in the cost of manufacturing a wafer without decreasing quality will lower the cost/power ratio and enable the wider availability of this clean energy technology.
The highest efficiency silicon solar cells may have an efficiency of greater than 20%. These are made using electronics-grade monocrystalline silicon wafers. Such wafers may be made by sawing thin slices from a monocrystalline silicon cylindrical boule grown using the Czochralski method. These slices may be less than 200 μm thick. To maintain single crystal growth, the boule must be grown slowly, such as less than 10 μm/s, from a crucible containing a melt. The subsequent sawing process leads to approximately 200 μm of kerf loss, or loss due to the width of a saw blade, per wafer. The cylindrical boule or wafer also may need to be squared off to make a square solar cell. Both the squaring and kerf losses lead to material waste and increased material costs. As solar cells become thinner, the percent of silicon waste per cut increases. Limits to ingot slicing technology may hinder the ability to obtain thinner solar cells.
Other solar cells are made using wafers sawed from polycrystalline silicon ingots. Polycrystalline silicon ingots may be grown faster than monocrystalline silicon. However, the quality of the resulting wafers is lower because there are more defects and grain boundaries, which results in lower efficiency solar cells. The sawing process for a polycrystalline silicon ingot is as inefficient as a monocrystalline silicon ingot or boule.
Yet another solution is to pull a thin ribbon of silicon vertically from a melt and then allow the pulled silicon to cool and solidify into a sheet. The pull rate of this method may be limited to less than approximately 18 mm/minute. The removed latent heat during cooling and solidifying of the silicon must be removed along the vertical ribbon. This results in a large temperature gradient along the ribbon. This temperature gradient stresses the crystalline silicon ribbon and may result in poor quality multi-grain silicon. The width and thickness of the ribbon also may be limited due to this temperature gradient. For example, the width may be limited to less than 80 mm and the thickness may be limited to 180 μm.
Producing sheets horizontally from a melt; may be less expensive than silicon sliced from an ingot and may eliminate kerf loss or loss due to squaring. Sheets produced horizontally from a melt also may be less expensive than a silicon ribbon pulled vertically from a melt. Furthermore, sheets produced horizontally from a melt may improve the crystal quality of the sheet compared to silicon ribbons pulled vertically or at an angle from a melt. A crystal growth method such as this that can reduce material costs would be a major enabling step to reduce the cost of silicon solar cells.
Horizontal ribbons of silicon that are physically pulled from a melt have been tested. In one method, a seed attached to a rod is inserted into the melt and the rod and resulting sheet are pulled at a low angle over the edge of the crucible. The angle, surface tension, and melt level are balanced to prevent the melt from spilling over the crucible. It is difficult, however, to initiate and control such a pulling process. First, the angle of inclination adjustment to balance gravity and surface tension of the meniscus formed at the crucible edge may be difficult. Second, a temperature gradient along the ribbon at the separation point between the sheet and the melt may cause dislocations in the crystal if the cooling plate is near this separation point. Third, inclining the sheet above the melt may result in stress at the freeze tip. This freeze tip may be where the sheet is thinnest and most fragile so dislocations or breaks in the sheet may occur. Fourth, a complex pulling apparatus may be needed to obtain the low angle.
Silicon used for electronics or solar cells may require high-quality silicon crystals. Solutes in the silicon feedstock, such as iron, have a negative effect on the solid state properties of the crystal. In one instance, minority carrier lifetime in devices made using such a sheet may be affected. Solutes also tend to segregate out of the crystal and can concentrate in the melt. This may lead to constitutional supercooling and dendritic growth. As the rate of solidification of the silicon increases, constitutional supercooling likewise may increase. Use of thermal gradients may create a convective flow in the melt and wash away any solutes from the freezing interface, but this also may cause thermal stress in the crystal. Thermal stress may lead to dislocations or defects.
Quartz may be used to surround the silicon melt in one embodiment. However, quartz may dissolve when in contact with this melt. Oxygen may be released into the melt and may form SiO, which results in oxygen in the silicon crystal that is formed. Oxygen may, for example, cause boron clustering in p-type solar cells and light-induced degradation (LID) of solar cell efficiency. Thus, there is a need in the art for an improved method of creating flows in the melt without thermal gradients and a method of creating flows in the melt while removing SiO.